1. Field of the Invention
This invention relates generally to non-dispersive infrared detectors for detecting small quantities of analyte gases. More specifically, the present invention relates to a method and apparatus for linearization of the response of a non-dispersive infrared detector in relation to the concentration or mass of the analyte gas.
2. Description of the Related Art
Infrared radiation detectors are used to monitor industrial process gas streams, in the medical field, for air pollution and respiratory measurements, and in laboratory applications.
Infrared radiation is an electromagnetic radiation. Any object whose temperature is above absolute zero radiates energy in relationship to the object's temperature. Infrared radiation detectors convert energy of infrared radiation into some other form of energy which may be processed more rapidly. In most cases, this conversion is from electromagnetic radiation energy into electrical energy. Infrared detectors may be classified as thermal detectors or photon detectors.
Thermal detectors are energy detectors, and their spectral response is dependent on the absorption properties of the detector. In such a detector, a thermal mass changes temperature as the impinging radiation changes. Several methods have been used to convert the temperature change of the mass into a usable electrical signal. These methods include the following: thermistor bolometer, thermopile detector, pyroelectric detector, and condenser microphone (Luft) detector. The majority of commercial infrared gas analyzers employ Luft detectors.
Photon detectors utilize the interactions of incident photons with the detector element. If the incident photon energy is sufficient to liberate an electron from the detector surface, then the so-called external photo-electric effect has occurred. When the incident photon merely liberates a free electron or a free hole or both in the body of the detector, then the internal photoelectric effect has occurred.
A fundamental difference between thermal detectors and photon detectors is that thermal detectors measure the rate at which energy is absorbed, while photon detectors measure the rate at which quanta are absorbed.
For many years, the absorption of radiation at certain wavelengths by gases has been used as a means of identifying and estimating them. When analyzing gasses, the vibrational spectra are of more practical interest than the rotational spectra. In general, infrared gas analysis methods may be either dispersive or non-dispersive. Dispersive methods are generally only suitable for laboratory use because of cost and fragility. Non-dispersive methods have been developed and are in widespread use.
Non-dispersive infrared detectors (NDIRs) consist of: (a) a suitable source of infrared radiation whose emission spectrum embraces the main absorption bands of the gases or vapors to be measured; (b) a measuring cell (or sample cell) through which the gas and radiation flow, containing the specimen of the gas or vapor stream to be analyzed, fitted with windows possessing suitable transmission properties; (c) means of restricting the wavelength range falling on the detector, such as an optical or gas filter; (d) means to modulate the infrared radiation from the source, such as a rotating chopper disk; (e) a detector block including a transducer in the form of an infrared detector to transform the infrared radiation into a corresponding electrical signal; (f) an amplifier for amplifying the detector signal; and (g) an output device.
Non-dispersive infrared detectors detect small quantities of gases that are non-symmetric in molecular structure in a matrix of diatomic or inert gas. Examples of non-symmetric gases that absorb energy in the infrared region include: carbon dioxide, carbon monoxide, nitrogen oxides, and sulfur dioxide. Examples of diatomic or inert gas matrices include: nitrogen, oxygen, helium, hydrogen, or a mixture of these (such as air). NDIRs can be made to be highly selective for one analyte over another, and are completely free of interference from a diatomic or inert gas matrix. This selectivity has made NDIRs the detectors of choice for certain analytical methods, including ambient air analysis for the above mentioned analytes and water analysis for total organic carbon.
Two drawbacks to using an NDIR in these and other analytical applications are: (1) relatively small dynamic range; and (2) inherent lack of linearity of the response. NDIRs typically can detect and reliably distinguish between about three orders of magnitude (i.e., a dynamic range of 1000), while other common detectors have dynamic ranges of five or six orders of magnitude. Part of the reason for this is because of a lack of linearity of the NDIR response.
As discussed above, the principle of operation of an NDIR is based on absorption of infrared energy by the analyte of interest due to the passage of the analyte through the measuring or sample cell, and the measurement of the subsequent decrease in light energy impinging the detector block.
One problem with NDIRs is that the response is not linear at higher points of analyte concentration, particularly at the upper end of the dynamic range of the NDIR. The relationships of the parameters and geometries involved cause the response to follow the Beer-Lambert Law to a large extent, but the response becomes attenuated at higher analyte concentrations. This aspect of the Beer-Lambert Law dictates an inverse logarithmic behavior of transmitted light energy as a function of the concentration of the analyte, so a linear response with increasing concentration is not anticipated. Due to various geometries of detectors, flows, and analytical methods, however, when the results of calibration runs are corrected according to the Beer-Lambert Law, linearity of the response is typically still not achieved.
In an attempt to provide a linear response, some NDIR designs have included analog electronic amplifiers which divide or segment the response into several steps at each level of analyte concentration within the dynamic range of the detector. For example, ten or eleven amplified steps may cover the entire range of the detector. These amplifiers are adjusted potentiometrically by trial and error in an attempt to cause the output of the NDIR at each level of analyte concentration to respond linearly with increasing concentration. The electronic amplification of NDIR responses has certain advantages, but the output is a non-continuous function because each segment is amplified separately.
At some point, increasing the portion of analyte gas in the flowing stream causes no further attenuation of light energy for a given NDIR system. This is because the analyte gas in the cell is already absorbing all of the radiation, and light transmittance at the detector cannot be reduced below zero. In other words, the detector cannot measure any further decrease in light transmittance through the cell. Near these levels, the response of the NDIR is very low for a given concentration change, as compared to the response with lower concentrations. At these levels, if the response is segmented and amplified, as discussed above, the amplifier responsible for bringing this raw signal into linearity with lower sections of the response curve must amplify the signal as much as 500 to 1000 percent. In contrast, lower sections of the raw signal need to be amplified only 10 to 20 percent. As a result, small errors in the raw signal get amplified excessively, and accuracy is diminished. In practice, the operator must determine the working upper limit of analyte concentration that can be determined and distinguished from slightly different concentrations, based on this asymptotic behavior and the extent of this type of tolerance.